This application relates generally to methods and apparatus for ground fault circuit interrupt (“GFCI”) detection. More particularly, this application relates to methods and apparatus for GFCI grounded neutral fault detection using multi-tone and/or multi-frequency signals to detect grounded neutral faults in electrical circuit branches of AC power systems.
Electrical circuit branches of single-phase AC power systems typically use electrical cables that include a line conductor and a neutral conductor coupled between a source and a load, with the neutral conductor grounded at the source. GFCI devices are installed in such circuit branches to interrupt power upon detection of ground current faults from the line conductor to ground at the load, as well as grounded neutral faults (e.g., low impedance connection faults) between the neutral conductor and ground at the load. GFCI devices provide safety protection from electrocution, and are primarily used in receptacles in kitchens, bathrooms and outdoor areas where water or moisture can pose a risk of electrocution. GFCI devices are also used in circuit breakers that protect these same areas.
GFCI devices typically use a differential current transformer to sense current imbalances in the line and neutral conductors resulting from ground leakage current from the line conductor returning to the source through an unintended ground circuit path other than the neutral conductor. To prevent injury from electrical shock, the GFCI device must initiate circuit interruption when the current differential in the line and neutral conductors is as low as 5 milliamps.
If a grounded neutral fault occurs, the differential current transformer may not detect the true magnitude of ground leakage current. In particular, because the neutral conductor is typically grounded at the source, a portion of the ground leakage current may return to the source through the neutral conductor. As a result, the current differential in the differential current transformer would not accurately correspond to the actual magnitude of the ground leakage current. Thus, a grounded neutral fault may desensitize the differential current sensor such that the GFCI device would trip only in response to considerably higher ground leakage current levels.
To address this issue, many previously known GFCI devices use a second transformer on the neutral conductor to detect grounded neutral faults. In such devices, if a low impedance connection fault exists between the neutral conductor and ground, the GFCI device forms an oscillator whose output signal is coupled to the differential current transformer using the second transformer. The oscillator signal is then used to detect grounded neutral faults. Upon detection of a grounded neutral fault, the GFCI device interrupts power in the AC power system.
Alternatively, some previously known GFCI devices use a single transformer to detect ground current faults and grounded neutral faults. For example, Howell U.S. Pat. No. 4,001,646, titled “Ground Fault Circuit Interrupter Utilizing A Single Transformer,” describes a GFCI device that uses a single transformer to detect ground current faults and grounded neutral faults. In particular, Howell uses a negative resistance network to form an oscillation signal that grows unless a low impedance connection fault exists between the neutral conductor and ground.
Other single-transformer GFCI devices, such as described in co-pending U.S. patent application Ser. No. 12/779,406, filed May 13, 2010, titled “METHODS AND APPARATUS FOR GROUND FAULT CIRCUIT INTERRUPT DETECTION USING A SINGLE TRANSFORMER,” which is incorporated by reference herein in its entirety for all purposes, detect grounded neutral faults by driving a single transformer using an AC source (e.g., an AC voltage source), and monitoring load current in the transformer.
In some previously known two-transformer and single-transformer GFCI devices, the oscillator or AC source operate continuously, periodically, or only when a grounded neutral fault occurs. Most previously known GFCI devices use an oscillator or AC source that uses a relatively high, single-frequency AC signal, and a comparator that trips if a threshold level is exceeded. Many such GFCI devices are susceptible to interference signals that may prevent detection of grounded neutral faults, or may be incorrectly detected as grounded neutral faults, unnecessarily interrupting AC power (commonly referred to as nuisance trips).
In particular, an AC power system may have load circuits that produce signals that interfere with GFCI devices. For example, motors (especially variable speed motors), rotary switches in light dimmers or fan speed controls, and fluorescent light fixtures may produce such interference signals. Typically, the interference signals are much higher in frequency than the AC power system frequency of 50 or 60 Hz. The circuitry in GFCI devices used to detect ground leakage current typically includes lowpass filtering that prevents high-frequency interference signals from disrupting the detector. However, a grounded neutral fault detector that uses a higher single-frequency AC signal detection method is susceptible to such high frequency interference signals.
Accordingly, improved GFCI devices are desirable.